getting to grips with aircraft performance 如何掌握飞机性能 2(11)

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At the long-range cruise Mach number, the specific range corresponds to 99% of the maximum specific range (Figure F4). Economically speaking, the 1% loss compared to the maximum specific range is largely compensated by the cruise speed increase due to the flatness of the curve.
MLRC > MMR
SRlong range = 0.99 x SRmax range

In relation to the Maximum Range Mach number, the long-range Cruise Mach number also decreases when weight decreases, as shown in Figure F5.

PA = constant weight . . LRC . weight = constant PA . . LRC .
2.1.3. Economic Mach Number (MECON)
Long-range Cruise Mach number was considered as a minimum fuel regime. If we consider the Direct Operating Cost instead, the Economic Mach number (MECON), can be introduced.
As indicated in §1.1, DOCs are made up of fixed, flight-time related and fuel-consumption related costs. As a result, for a given trip, DOC can be expressed as:
DOC = C + C ..F + C ..T
CF T
That is:
CC = fixed costs
CF = cost of fuel unit
.F = trip fuel
CT = time related costs per flight hour
.T = trip time

As DOCs are calculated per nautical mile, it is possible to plot fuel-related costs, flight-time related costs, and direct operating costs based on Mach number (Figure F6).

Minimum fuel costs correspond to the Maximum Range Mach number. The minimum DOC corresponds to a specific Mach number, referred to as Econ Mach (MECON).
PA = constant weight . . MECON . weight = constant PA . . MECON .
The MECON value depends on the time and fuel cost ratio. This ratio is called cost index (CI), and is usually expressed in kg/min or 100lb/h:
Cost of time CT
Cost Index (CI) = =
Cost of fuel CF
When CT is fixed and CF increases, it becomes interesting to decrease fuel consumption. Therefore, when CI decreases, Econ Mach decreases.
CI . . MECON .
CI . . MECON .

The extreme CI values are:
.  
CI = 0: Flight time costs are null (fixed wages), so MECON = MMR (lowest boundary).

.  
CI = CImax: Flight time costs are high and fuel costs are low, so MECON = MAX SPEED in order to have a trip with a minimum flight time. The maximum speed is generally (MMO - 0.02) or (VMO - 10kt).

For instance, a cost index of 30 kg/min means that the cost of one flightminute is the same as the cost of 30 kg of fuel. This does not mean the fuel flow is 30 kg/min.

2.1.4. Constant Mach Number
The aircraft is often operated at a constant Mach number.

Nevertheless, as the aircraft weight decreases, the gap between the selected Mach and the MMR increases. As a result, fuel consumption increases beyond the optimum.
3. ALTITUDE OPTIMIZATION
3.1. Optimum Cruise Altitude
3.1.1. At a Constant Mach Number
In examining SR changes with the altitude at a constant Mach number, it is apparent that, for each weight, there is an altitude where SR is maximum. This altitude is referred to as “optimum altitude” (see Figure F8).
　
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